Press machines are roughly classified into two types: hydraulic press machines and mechanical press machines. The hydraulic press machines use a hydraulic pressure as a drive source to drive a slide. The mechanical press machines drive a driving mechanism (for example, a crank mechanism) with an electric motor to vertically move a slide connected to the driving mechanism so as to perform press working.
The hydraulic press machine (hereinafter, also referred to as “hydraulic press”) is advantageous in ease in changing a stroke, and easy adjustment and retention of a pressure force. However, the hydraulic press is disadvantageous in a large energy loss due to heating and cooling and low productivity due to a low processing speed because of the use of the hydraulic pressure. On the other hand, the mechanical press machine (hereinafter, also referred to as “mechanical press”) has drawbacks in terms of adjustment and retention of the pressure force. However, the mechanical press has a high processing speed, and therefore is overwhelmingly advantageous in terms of productivity. Thus, the mechanical presses are used in 95% or larger of press working for mass production.
The press working includes various types of processing such as punching, drawing, and forging. Among the various types of processing, closed-die forging, some types of squeezing, and powder molding require the precise adjustment of the pressure force. Thus, even though the mechanical press is overwhelmingly advantageous in terms of productivity, the mechanical press is not available for the aforementioned types of processing and the hydraulic press has to be used in some cases.
On the other hand, the conventional mechanical presses generally use the following mechanism. A flywheel is driven by an electric motor. A stored energy of the flywheel is used to drive an eccentric mechanism such as a crank mechanism (through an intermediation of a reducer as needed) with a turning force of the flywheel so as to vertically drive a slide connected to the eccentric mechanism. In recent years, however, an electric servo press which directly drives the eccentric mechanism such as the crank mechanism (through an intermediation of the reducer as need) by a servo motor without using the flywheel has been launched into the market and used. The use of the electric servo press expands the range of processing of the mechanical press and therefore, increases at a rapid rate because the electric servo press enables free motion to be set for the slide.
For example, Japanese Patent Application Laid-Open NO. H10-277797 describes a multi-stage motion controller using a servo press. Japanese Patent Application Laid-open NO. H10-277797 describes that an output torque of a servo motor is adjusted by controlling a driving current of the servo motor during a multi-stage motion process to obtain a pressure force of the slide according to set motion data. The multi-stage motion controller described in Japanese Patent Application Laid-Open NO. H10-277797 does not use the rotary mechanism such as the crankshaft but uses a hydraulic cylinder, a linear drive electric motor, or a power converter using a ball screw, as a slide driving mechanism so as to vertically drive the slide.
Japanese Patent Application Laid-open NO. 2003-181698 describes an electric servo press using a crankshaft. When the position of a slide reaches a press-working area, a control system, of a servo motor is switched from a positional control system to a pressure-force control system to control a pressure force of the slide. Further, the relation between a torque of the electric servo motor and the pressure force of the slide varies depending on the position of the crankshaft because of the structure of the electric servo press using the crankshaft. In view of the relation as described above, a relational expression of the torque of the servo motor and the pressure force of the slide depending on an angle of rotation of the crankshaft is used to control the output torque of the servo motor so as to obtain a set pressure force of the slide.
Further, Japanese Patent Application Laid-Open NO. 2003-154498 describes a technology of performing pressurization f or a set time period with a maximum pressure force. Japanese Patent Translation Publication NO. 2009-505834 (corresponding to a translation of WO 2007/022755) describes a technology of eliminating the effects of an increase or decrease in acceleration rate of each movable portion for the precision of the pressure force of the slide.
The linear drive electric motor (linear motor) described in Japanese Patent Application Laid-open NO. H10-277797 is capable of performing motions as those performed by the hydraulic press. On the other hand, it is difficult to obtain a large thrust in view of the structure. In addition, a coil-side unit and a permanent magnet-side unit which has a large attracting force are independent of each other. Therefore, the linear drive electric motor has difficulty in handling the permanent-magnet side unit at the time of incorporation of the permanent-magnet side unit into the press machine. Further, it is not easy either to increase the torque by using the reducer to obtain a large thrust as in the case where the rotary electric motor is used.
Similarly to the linear motor, even in the case of the power conversion mechanism using the ball screw, which is described in Japanese Patent Application Laid-Open NO. H10-277797, it is difficult to obtain a large thrust due to restrictions in view of the mechanism. The pressure force at the time of molding of a product is directly applied to the ball screw to increase a friction force. As a result, there is a fear of causing the abrasion of the ball screw. Therefore, in the actual conditions, it is difficult to employ the linear drive electric motor and the mechanism using the ball screw for a large press machine which is required to have a large pressure force.
On the other hand, the press machine using the crankshaft or the eccentric shaft is relatively easy to fabricate even though the press machine is required to have a large pressure force. Thus, a large number of press machines ranging in size from small to large are fabricated.
In Japanese Patent Application Laid-Open NO. 2003-181698, the press machine using the crankshaft is described. The press machine described in Japanese Patent Application Laid-Open NO. 2003-181698 controls the torque of the servo motor by using the relational expression of the torque of the servo motor and the pressure force of the slide, which depends on the angle of rotation of the crankshaft. Therefore, it seems that the precise control of the pressure force can be performed.
In theory, however, the press machine using the eccentric mechanism can generate an infinite pressure force of the slide at bottom dead center (hereinafter, abbreviated as BDC). Therefore, a slight fluctuation in torque of the servo motor appears as a large change in pressure force in the vicinity of BDC.
Therefore, a method of adjusting the output torque by the control of the current of the servo motor so as to control the pressure force of the slide has a characteristic in that an error in output torque of the servo motor is amplified as the slide comes closer to BDC, which also increases an error in pressure force of the slide.
Further, in theory, the pressure force becomes infinite at BDC as described above. The effects of a frictional force of a sliding portion for driving the eccentric mechanism cannot be neglected any more. Thus, in practice, it is difficult to precisely control the pressure force only by controlling the torque of the servo motor.
If the setting is performed so that the speed is changed with a high degree of freedom, which is a characteristic of the electric servo press, in the vicinity of BDC, the effects of moment of inertia of the reducer mechanism or the crankshaft on the pressure force of the slide are generated in an amplified manner. Therefore, control accuracy is further lowered. As a result, load control scarcely functions in the vicinity of BDC.
The press using the eccentric mechanism such as the crankshaft is capable of generating a large pressure force in the vicinity of BDC. Therefore, an area of the vicinity of BDC is forced to be positively used as the press-working area. Therefore, the control has low accuracy in the most frequently used area over the stroke of the slide of the press. Accordingly, in the actual conditions, the press working with high accuracy cannot be satisfactorily realized.